1. Field of the Invention
The present invention relates to an oxygen sensor control apparatus which detects oxygen concentration within exhaust gas by use of an oxygen sensor.
2. Description of the Related Art
Conventionally, an oxygen sensor has been disposed in an exhaust passage (exhaust pipe) of an internal combustion engine of an automobile or the like so as to detect the oxygen concentration of exhaust gas for the purpose of controlling the air-fuel ratio of the engine. Such an oxygen sensor includes, for example, a gas detection element which has at least one cell composed of an oxygen ion conductive zircona body and a pair of electrodes formed thereon. However, such an oxygen sensor has a problem. That is, the detection accuracy of the oxygen concentration changes because of variation in output characteristics among individual oxygen sensors and deterioration of each oxygen sensor with time. In order to solve such a problem, a technique has been proposed of performing ambient atmosphere correction; i.e., a technique of stopping supply of fuel to an internal combustion engine, calculating a correction coefficient when the exhaust passage is assumed to be substantially completely filled with ambient atmosphere, and calibrating (correcting) the relation between an output value of an oxygen sensor and oxygen concentration (see, for example, Patent Document 1).    [Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2007-32466
3. Problems to be Solved by the Invention
However, even in a state in which the supply of fuel to the internal combustion engine is stopped (so-called “fuel cut”), the output value of the oxygen sensor fluctuates because it pulsates as a result of operation of the internal combustion engine, and noise is superimposed on the output thereof. In order to reduce the influence of such fluctuation, a method may be employed of determining a timing for calibration when the output value of the oxygen sensor deviates, by a predetermined amount or greater, from a reference value successively in a predetermined number (two or more) of fuel cut operations, and computing a correction coefficient at that timing. In the case where the oxygen sensor deteriorates with time, the accuracy in detection of oxygen concentration by the oxygen sensor deteriorates gradually. Therefore, after the output value of the oxygen sensor has gradually deviated from a predetermined range, a correction coefficient is computed upon performing a predetermined number of fuel cut operations, and the relation between the output value of the oxygen sensor and the oxygen concentration of exhaust gas is calibrated. The correction coefficient is not computed in a period during which the predetermined number of fuel cut operations are performed. However, since the output value of the oxygen sensor is close to the predetermined range, the oxygen concentration of exhaust gas flowing through the exhaust pipe can be detected relatively accurately.
Meanwhile, in the case where an oxygen sensor attached to an internal combustion engine is replaced with a new one, the output value of the new oxygen sensor may greatly deviate from the predetermined range due to a difference (variation attributable to an individual difference) in characteristics or a difference in degree of deterioration between the old and new oxygen sensors. Also, in the case where a correction coefficient calculated for the old oxygen sensor is stored, the oxygen concentration is calculated from that correction coefficient and the output value of the new oxygen sensor. Therefore, the accuracy in detecting the oxygen concentration of exhaust gas flowing through the exhaust pipe deteriorates. An operator who has changed the oxygen sensor may compute a correction coefficient for the new oxygen sensor so as to avoid the above-mentioned problem. However, such computation is troublesome for the operator. Since a control apparatus for controlling the oxygen sensor has a function of computing such a correction coefficient, it is desirable to compute such a correction coefficient by making use of that function. In view of the above, the control apparatus must calculate an optimal correction coefficient for the new oxygen sensor as soon as possible, and calibrate the relation between the output value of the oxygen sensor and the oxygen concentration of exhaust gas. However, the correction coefficient is not computed unless a predetermined number of fuel cut operations are performed, which increases a period of time during which the accuracy in detecting the oxygen concentration of exhaust gas is low.